net: more accurate IPv4-in-IPv6 API test

R=rsc
CC=golang-dev
https://golang.org/cl/4172045

src/pkg/net/iprawsock.go

@@ -245,7 +245,7 @@ func hostToIP(host string) (ip IP, err os.Error) {
 			err = err1
 			goto Error
 		}
-		addr = ParseIP(addrs[0])
+		addr = firstSupportedAddr(addrs)
 		if addr == nil {
 			// should not happen
 			err = &AddrError{"LookupHost returned invalid address", addrs[0]}

src/pkg/net/ipsock.go

@@ -18,19 +18,34 @@ import (
 // Unfortunately, we need to run on kernels built without IPv6 support too.
 // So probe the kernel to figure it out.
 func kernelSupportsIPv6() bool {
-	// FreeBSD does not support this sort of interface.
-	if syscall.OS == "freebsd" {
+	s, err := syscall.Socket(syscall.AF_INET6, syscall.SOCK_STREAM, syscall.IPPROTO_TCP)
+	if err != 0 {
 		return false
 	}
-	fd, e := syscall.Socket(syscall.AF_INET6, syscall.SOCK_STREAM, syscall.IPPROTO_TCP)
-	if fd >= 0 {
-		closesocket(fd)
+	defer closesocket(s)
+
+	la := &TCPAddr{IP: IPv4(127, 0, 0, 1)}
+	sa, oserr := la.toAddr().sockaddr(syscall.AF_INET6)
+	if oserr != nil {
+		return false
 	}
-	return e == 0
+
+	return syscall.Bind(s, sa) == 0
 }
 
 var preferIPv4 = !kernelSupportsIPv6()
 
+func firstSupportedAddr(addrs []string) (addr IP) {
+	for _, s := range addrs {
+		addr = ParseIP(s)
+		if !preferIPv4 || addr.To4() != nil {
+			break
+		}
+		addr = nil
+	}
+	return addr
+}
+
 // TODO(rsc): if syscall.OS == "linux", we're supposd to read
 // /proc/sys/net/core/somaxconn,
 // to take advantage of kernels that have raised the limit.
@@ -208,7 +223,7 @@ func hostPortToIP(net, hostport string) (ip IP, iport int, err os.Error) {
 				err = err1
 				goto Error
 			}
-			addr = ParseIP(addrs[0])
+			addr = firstSupportedAddr(addrs)
 			if addr == nil {
 				// should not happen
 				err = &AddrError{"LookupHost returned invalid address", addrs[0]}